If it still mounts and doesn't throw any errors probably still keep it as a redundant cold storage just don't add anything to it anymore. That way if your new backup drive dies because of x reason you still have a chance to recover some data.

Edit: this is assuming its nothing too sensitive where you one want a certain amount of copies in the world and its not under warranty anymore. If it is sensitive, you can try removing the drive from the enclosure, this is called shucking, tape it up with a few layer of whatever tape you have, then either take a drill or a pickaxe to it to destroy the shiny disc of metal inside, aka the platter make sure you do this outside. If its under warranty, securely erase it and get it sent off to be replaced by WD